From orbiting lasers to metal rods that strike from the heavens, the potential to wage war from space raises startling possibilities and serious problems

12 June 2018--The world awakens to an international crisis: officials at the Tokyo airport have detained a foreign airliner suspected of carrying illegal arms. The aggressive and threatening response from the plane's country of origin, a "rogue" state believed to possess both nuclear and biological weapons, adds credibility to the suspicion. Hamstrung by its rogue status, the country's economy has been in free fall for decades, and with this latest incident, it's widely feared that the country will launch a nuclear attack against Japan. U.S. satellites report escalating activity at the country's rocket-launch facility; other U.S. intelligence indicates that three intermediate-range missiles are being fueled and are within a 15-minute launch window. No air-, sea-, or land-based military system is available to respond in time. The U.S. president demands that the country cease and desist immediately but receives no response. Five minutes later, the U.S. Strategic Command activates a heretofore undisclosed space-based laser; within minutes, it incinerates the launch facility's command and control center, thus narrowly averting a catastrophe.

Today, such a scenario is science fiction, but it--or something like it--could become reality within the next decade or two. The irony is that the economic and political price the United States would have to pay to bring about such a system, even if it could be done, might well outweigh its military benefit.

No country today is known to have weapons deployed in space, and many countries oppose their development. However, at least some U.S. Pentagon officials have been arguing that the United States must now, after decades of debate, develop and deploy offensive space weapons. In fact, over the past 10 years, the U.S. government has spent billions of dollars researching and testing such weapons. If deployment became official U.S. policy, such a step would have profound--and, we feel, profoundly negative--implications for the balance of global power.

The United States itself, our analysis suggests, would discover that the military advantages that might be gained from space-based weapons are outweighed by their political and economic costs. Deploying such weapons would also create new, asymmetric vulnerabilities to U.S. armed forces, as we will describe in this article. In addition, it would be a significant political and strategic departure from 50 years of international law and diplomatic relations.

The U.S. and North Atlantic Treaty Organization (NATO) militaries already make extensive use of space-based systems. Satellites revolutionized conflicts such as Operation Iraqi Freedom, letting U.S. aircraft fly one-third the number of sorties and use one-tenth the number of munitions that they had expended just 10 years earlier in the Persian Gulf War. That economy was largely due to the great increase in accuracy offered by space systems.

Satellites are now routinely used to detect, identify, locate, and track targets. They also provide mobile, secure communication links between military control centers and theaters of operation; near-real-time imaging; signals intelligence; and meteorological data. And, of course, the constellation of Global Positioning System (GPS) satellites ensures that military personnel need never be lost amid a war's chaos.

With capability, however, has come reliance. In the words of one U.S. Air Force space official, space systems are now "woven inextricably" throughout the military capabilities of the United States and its allies. Moreover, dependency on space is increasing. By 2010, the U.S. military expects, it will need twice the capacity of its existing space-based infrastructure--in everything from the number of images per day acquired from spy satellites to the bandwidth carried by communications satellites.

Without a doubt, the exploitation of space has helped the U.S. military remain the most technologically advanced fighting force in the world. At the same time, though, it has made that force deeply vulnerable to an attack on its satellites and other space-based systems. What's more, the means to disable or disrupt this valuable and complex machinery are well within the reach of even technologically unsophisticated adversaries.

Indeed, with some U.S. military planners advocating the development of what would be the first-ever space-based systems for offensive operations--what the military refers to as force projection

--the country finds itself fast approaching a crossroads. Space, these planners assert, will usher in a revolution in global warfare, with U.S. space-based weapons delivering destructive force to any point on the globe within minutes, and without the risk or cost of sending troops.

Realizing the growing strategic value of space, in January 2001 a congressionally mandated space commission headed by incoming Secretary of Defense Donald H. Rumsfeld urged the United States to maintain the option of weaponizing space, identifying three potential missions for space weapons:

Protecting existing U.S. systems in space.

Denying the use of space and space assets to adversaries.

Attacking from space a target anywhere on land, at sea, or in the air.

In the four years since the Rumsfeld commission released its conclusions, the report has continued to guide U.S. policymaking in this arena. For instance, the U.S. Air Force last year outlined a series of potential space weapons initiatives as part of its 176-page Transformation Flight Plan. Among the weapons described were space- and ground-based lasers, antisatellite missiles, and a futuristic constellation of orbiting high-power radio frequency transmitters capable of disrupting or disabling electronics. A press statement that accompanied the report's release in February 2004 described it as "a road map to the future."

The Idea Of Putting Weapons In Space is not new. Beginning in the 1960s, at a time when satellites were still quite rare, the former Soviet Union and the United States both tested antisatellite weapons. Despite several decades of development, however, neither country managed to deploy any such weapons. Then, during the Reagan administration, supporters of the Strategic Defense Initiative advanced proposals ranging from space-based lasers to "Brilliant Pebbles," numerous small orbiting projectiles to be fired at ballistic missiles in hopes of destroying them [see sidebar, "Missile Defense from Space"].

Considerable research netted no system worth deploying. Though such systems were positioned as defensive in nature, the line between offensive and defensive space weaponry is more philosophical than technological: the same laser that could be trained on a rogue missile could easily target a commercial satellite instead. Likewise, the technological problems that plagued defensive space weapons will also apply to new offensive designs.

Critics of space weapons have long insisted that developing and deploying space weapon systems--if feasible at all--would be prohibitively expensive and technologically difficult. The majority of the international space-faring community calls instead for a perpetuation of the status quo: the use of space to support terrestrial military activities through communications, reconnaissance, navigation, and even weapon guidance, but not for direct application of force. In other words, the militarization of space is acceptable; the weaponization is not.

Now, as the U.S. national security community nears a decision point, policymakers are split on several fundamental questions:

Can space weapons effectively mitigate the existing vulnerabilities of U.S. and other satellites and space systems?

Will space weapons be better than terrestrial alternatives at projecting force and denying adversaries the use of space?

Will expected gains from space weapons outweigh financial, strategic, and political costs?

Assuming for the moment that space weapons would further U.S. interests, but taking into account that several other countries also have the ability to deploy them, should the United States be the first to do so?

What Is A Space Weapon? As commonly defined, it is a system designed to project destructive force between Earth and outer space or within space itself. Antisatellite weapons, space-based lasers, space-based platforms that fire projectiles, and ground-based lasers that rely on orbiting mirrors to reflect beams to space or back down to Earth--all fit the definition. On the other hand, intercontinental ballistic missiles, ground-based electromagnetic jammers aimed at satellite signals, and explosives used to attack satellite ground stations are not considered space weapons.

For the most part, space weapons can be classified into four categories: directed-energy weapons, kinetic-energy weapons, conventional warheads delivered to or from space, and microsatellites.

A directed-energy weapon uses a beam of electromagnetic energy--whether laser light or high-powered radio waves--to destroy a target. In the case of a laser, the beam heats a target until it melts or catches fire. A radio-wave weapon stimulates the target's electronic circuits until they are inoperable [see The Dawn of the E-Bomb," IEEE Spectrum, November 2003].

The most widely discussed directed-energy weapon is the space-based laser (SBL), an orbiting system that would use powerful lasers with large mirrors to focus energy on a selected target on Earth, producing damaging or destructive levels of heat. Over the past decade, the Pentagon has spent roughly US $750 million on SBL research, funded primarily by the Air Force and the Ballistic Missile Defense Organization (now the Missile Defense Agency).

Various components have been tested on the ground and in the lab, including a megawatt-class chemical laser and the apparatus for pointing and controlling the beam, but the full system has yet to be tested in orbit. Although the U.S. Congress suspended funding in 2003 and called for a review of the program, the concept remains very much alive.

Directed-energy weapons propagate their energy at the speed of light, so their effects begin with no appreciable delay beyond the time necessary to acquire a target and point the laser. However, to have the desired result, the beam must remain on target for some time. For example, to attack a ballistic missile at a range of 3000 km, a space-based 3-MW laser with a 3-meter-diameter mirror stationed 1000 kilometers above Earth's surface, in low Earth orbit (LEO), requires an impractical 2 hours and 13 minutes to burn through the rocket casing ; a 30-MW laser with a 10-meter-diameter mirror in the same orbit and at the same range would take a more reasonable 80 seconds [see Illustration, "Light Saber"]. For comparison, the entire flight of an intercontinental ballistic missile, from launch to impact, would last only about 45 minutes.

Light Saber:

Space-based lasers would destroy targets by heating them with a powerful beam generated by a chemical reaction between hydrogen and fluorine. However, many such lasers would be required for global coverage; clouds and smoke block the beam; and keeping the beam on target long enough to cause damage is difficult.

"Burn" time aside, directed-energy weapons' speed-of-light propagation cannot be matched by any other weapon. This feature suits them well for time-critical targets or those in remote locations or beyond the reach of conventional forces, such as the launch site described in the opening scenario. But even if the target's location is known precisely, the laser is useless if clouds or smoke intervene.

Kinetic-energy weapons destroy targets by smashing into them at high speed (they are not explosive). According to basic Newtonian physics, the impact energy increases linearly with the projectile's mass but as the square of its impact velocity. Because collision speed is comparable to orbital or missile speed, an inert projectile would be sufficiently destructive--assuming it finds its target. Such velocities would also help the projectile elude countermeasures and defenses, penetrate armor, and reach buried targets.

Hypervelocity Rod Bundles are a leading candidate. More colloquially known as Rods From God, they are long, slim, dense metal rods, typically of tungsten or uranium, each weighing perhaps 100 kilograms and deployed from an orbiting platform. Once a rod is released by the platform, a large two-stage rocket would bring it to a stop, after which orbital dynamics determine the projectile's trajectory to a terrestrial target [see Illustration, "Space Arrows"]. The slender rods would eventually reach a speed of several kilometers per second if dropped from LEO, their length facilitating the penetration of hard or buried targets.

Space Arrows:

Rods of tungsten, stored on an orbiting platform [top], would be released to strike buried targets on Earth. However, each rod would take several minutes to reach its target and would be difficult to steer, limiting the weapon to attacking fixed positions. There is also an upper bound on the rods' velocity, which means their destructive force would be similar to that of cheaper conventional explosives.

Because the rods' trajectory paths from LEO would be many hundreds of kilometers long, they would require about 5 minutes to reach their targets, so it would be difficult to use them against moving objects. Since no target is likely to be directly under the platform's orbital path, each rod would have to be equipped with a rocket or some other means to move it from that path. Also, the rods would need shielding to keep them from burning up during reentry. The shielding and rocket both add weight and thus increase the cost of putting these weapons into orbit in the first place. Once the rod has reentered Earth's atmosphere, it could be maneuvered by shifting an internal mass or by ejecting gas.

How destructive could such a weapon be? A 100-kg rod of tungsten falling from an altitude of 460 km and reaching an impact velocity of roughly 3 km/s would have the destructive force of a similar amount of conventional high explosives delivered by bomb or missile. The rod would be more effective than conventional high explosives at penetrating to a buried target, because the rod's force would be concentrated and directed in the line of motion. Higher orbits would deliver greater energies but would take even longer to strike a target--about 6 hours, for instance, from geosynchronous orbit.

Conventional warheads delivered from space are yet another candidate for the space weapons arsenal. (A conventional intercontinental ballistic missile, or ICBM, which also delivers bombs from above, spends relatively little time in space during its trajectory and is not a space weapon.) One proposal for delivering large quantities of conventional explosives is the Common Aero Vehicle (CAV), a robotic hypersonic aircraft much like a miniature space shuttle. Championed by the U.S. Air Force and the Defense Advanced Research Projects Agency, the Pentagon's entrepreneurial R and D wing, based in Arlington, Va., the CAV would be launched into orbit by a land-based missile, aircraft, or some as-yet-undeveloped military space plane [see Illustration, "Orbital Bomber"].

Orbital Bomber:

A robotic hypersonic aircraft could carry large amounts of conventional explosives to terrestrial targets. However, basing such a system in space would be prohibitively expensive.

To attack, a CAV would come down from orbit, reenter Earth's atmosphere, and maneuver to its target at speeds as high as Mach 25. Like the ICBM, the CAV would have one political edge over conventional aircraft: because the vehicle would reenter sovereign airspace only over the target country, the attacker would need no permission to fly over other countries.

CAVs could strike hard and deeply buried targets, naval bases, surface combatants, massed forces, mobile targets, air bases, and military and civilian infrastructure, to name a few examples. To strike a target on the other side of the globe would take about 45 minutes. Other advantages of such rapid strikes include having global reach, the ability to bypass enemy air defenses, and the absence of risk to pilots or support staff. However, in comparison with existing airborne alternatives and missile payloads, the CAV would be costly, and development would take many years.

Microsatellites , of all the space weaponry now being developed, are the closest to operational use. Microsatellite "mines" that would blow up or collide with other satellites could be ready to deploy within a few years of a decision to do so. If that decision has already been made, deployment could occur within days of a triggering event.

These small, maneuverable satellites would be launched into space on rockets or from larger satellites. Once in orbit, they would be self-powered and -guided. Microsatellites are being developed today for surveillance, inspection, and other nonoffensive tasks, but they could also be used as weapons--for example, to attack a far larger and more valuable satellite by blowing up or simply colliding with it at high speed. With compact communications, guidance, control, sensing, and propulsion systems, a microsatellite might weigh only tens or at most hundreds of kilograms, compared with its full-sized cousins weighing thousands of kilograms or more [see Illustration, "Mobile Mine"].

Mobile Mine:

Cheap and maneuverable, a microsatellite could creep up on an enemy satellite and either explode or simply collide with it. But if the United States deployed such weapons, it could open the floodgates to similar threats to U.S. military and commercial satellites.

Drawing a line between peaceful and hostile microsatellites may be impossible. In January 2003, the U.S. Air Force demonstrated its XSS-10 microsatellite, which repeatedly maneuvered to within 35 meters of a target to take photographs. Had it been equipped with a gun instead of a camera, it could have destroyed the target.

Within a few months, the Air Force is due to launch the follow-up XSS-11, designed for "rendezvous and proximity operations"--that is, meeting with other satellites to perform inspections, maintenance, and the like. However, as an unnamed U.S. defense official candidly acknowledged in an interview with Inside the Pentagon in December 2003, the XSS-11 could also be used as an antisatellite weapon.

The United States is not unique in its microsatellite capability. Over the last decade, for instance, researchers at the University of Surrey, in Guildford, England, have successfully launched a range of nonmilitary microsatellites, often in partnership with teams from other countries, and have orbited and tested a "nanosatellite" weighing less than 10 kg.

In a sense, microsatellites are as old as space exploration itself. Sputnik-I, weighing in at 84 kg, was technically a microsat, and many of the spacecraft that followed in those early years were similarly small. In the five decades since then, researchers worldwide have steadily refined microsat components, helped tremendously by the general shrinking of sensors and circuitry for computers and communications. At present, a microsat's guidance and control systems can be miniaturized to considerably less than 1 kg, and can derive both propulsion and power from solar cells, thus reducing weight and launch costs.

Although microsatellites are perceived primarily as a threat to satellites in LEO, they could be adapted to attack assets in geosynchronous orbit as well. A space mine would be effective only if it were orbiting very close to its quarry, in an almost identical orbit. The space mine would not need to be deployed covertly; there would be no means of destroying or disabling the mine without also risking the destruction of its much more valuable target, so the mine poses a similar threat whether its presence is known or unknown.

Should The United States , or any nation for that matter, weaponize space? The answer depends not simply on the capabilities and limitations of proposed space weapons but also on the military objectives. The Rumsfeld commission laid out three objectives in which space weapons might play a role: to defend existing military capabilities in space; to deny adversaries the military benefit of space; and to attack adversaries from or within space.

The last objective is perhaps the most alluring: the prompt and deadly projection of force anywhere on the globe. The psychological impact of such a blow might rival that of such devastating attacks as Hiroshima. But just as the unleashing of nuclear weapons had unforeseen consequences, so, too, would the weaponization of space. What's more, each of the leading proposed space weapons systems has significant physical limitations that make alternatives more effective and affordable by comparison.

Except for those in geosynchronous orbit, all satellites are in motion relative to Earth. Space weapons would be no different. A satellite in LEO, for example, circumnavigates the globe roughly every 90 minutes. Traveling at high speed relative to the ground, each satellite has a limited window during which to strike a particular ground location--from LEO, typically 1 or 2 minutes, during which time the satellite moves 500 to 1000 km.

A reasonable response time, then, means having an overlapping constellation of many satellites. A satellite capable of destroying a target up to 3000 km away could cover a circular area of 28 million square kilometers, or about one-18th of Earth's total area. In theory, 18 identical laser-weapon satellites would be needed to cover every location on Earth. Unfortunately, the circular coverage areas of the individual satellites would provide overkill at some points and no effectiveness at others. For example, in a 2002 Air Force-sponsored RAND report, "Space Weapons, Earth Wars," Bob Preston and his coauthors describe how a constellation of twenty-four 5-MW hydrogen-fluorine lasers with 10-meter-diameter mirrors would usually be able to destroy two to four ballistic missiles launched simultaneously from a small area, but if one missile was launched every 5 minutes or so, the constellation would be able to destroy just one.

For lower-power lasers, the number of satellites escalates. For 1-MW beam power, 120 satellites could kill a launch of four missiles most of the time, but occasionally would be able to destroy only three. The main point is that many weapons (of any type) need to be orbiting to ensure that at least one weapon is within range to strike any possible target at any given time.

An additional challenge for space-based lasers is their vulnerability to countermeasures. As we have noted, even the highest-power lasers do not penetrate clouds or smoke, and some wavelengths cannot penetrate Earth's atmosphere, including those used by the HF laser currently proposed for space-based missile defense. For ground targets, smoke pots could disrupt an attack already in progress.

Vulnerability is increased by the need to keep the laser on target for tens of seconds at least. The target could move in an unpredictable path or simply be covered with a reflective coating or paint, which could increase the time required for a successful kill by a factor of 10 or more. A layer of titanium oxide powder, for instance, could reflect 99.9 percent of the incident laser energy. Even a shallow pool of dyed water would offer serious protection for structures. Since a 20-MW laser boils water at a rate of 10 kg/s, a pool of water about 3 centimeters deep on the flat roof of a two-car garage would protect against 100 seconds of illumination by a space-based laser. This all adds up to abundant opportunity to thwart laser weapons.

Meanwhile, the laser would be burning its supply of hydrogen and fluorine at a rate of 500 kg/s. Over the course of 100 seconds, it would consume 50 tons of fuel, for which the launch costs alone are about half a billion dollars.

The issue of energy requirements warrants a closer look. Today, the most efficient high-power lasers typically consume 2 to 3 kg of chemical fuel per megawattsecond. So a pulse of 20 seconds from a 10-MW laser corresponds to about 400 to 600 kg of fuel per target in the absence of any countermeasures. At current launch costs of some $22 000/kg into low Earth orbit, each 20-second laser shot would cost approximately $11 million. For a constellation of 17 lasers, each loaded with a 12-shot capacity, the launch cost to maintain on-orbit fuel alone would exceed $2 billion. Weigh that against a stock of highly effective $6 smoke grenades, a stray cloud, or a 3-cm-deep pool of water, and this multibillion-dollar weapon system starts to look like a poor investment.

If lasers are prohibitively expensive, might long tungsten rods used as high-speed penetrators be a relative bargain? Not really. To guarantee that a single target (located near the equator, to take the easiest case) could be attacked at will, and not only when a single orbiting rod happened to pass overhead, a distributed constellation of some 40 rods would be necessary, with launch costs totaling some $8 billion.

The additional problems of targeting at supersonic speeds and coping with the intense heat of reentry demand extremely advanced, and therefore costly, technologies. Although one can steer the rod by shifting its center of mass, one would still need to obtain error signals to guide the penetrator to the target. Communicating with the penetrator is complicated by the fact that the surrounding air is heated into a radiopaque plasma, obstructing even the reception of GPS navigation signals. Although none of these problems is insoluble, they defy inexpensive solutions.

For attacking hardened or deeply buried targets, the long rods would not outperform existing missiles equipped with conventional penetrating warheads. That's because the physics of high-velocity impacts limits the penetration depth; basically, too much energy at impact causes the projectile to distribute its energy laterally rather than vertically. Tests done since the 1960s by Sandia National Laboratories, in Albuquerque, N.M., confirm that for even the hardest rod materials, maximum penetration is achieved at a velocity of about 1 to 1.5 km/s.

Above that speed, the rod tip liquefies, and penetration depth becomes essentially independent of impact speed. Therefore, for maximum penetration, the long rods would need to be slowed to about 1 km/s, thereby delivering only one-ninth the destructive energy per gram of a conventional explosive--or about 1.5 percent of the potential energy the rod had in LEO. The wasted energy would be immense, and the effort, cost, and complexity of such an orbital system would be entirely out of proportion to the results.

For soft targets on the surface, such as aircraft, ships, or even tanks, the United States already has many quicker, simpler alternatives to space-based kinetic energy systems such as long rods. Explosives delivered by long-range cruise missile, ICBM, or submarine-launched ballistic missile are all more attractive options.

The space-based common aero vehicle also comes out a loser in comparison with weapons delivered by ICBM or shorter-range missile. Although the CAV may take only 45 minutes from launch to detonation, that would be preceded by as much as 12 hours for the target to come into range. Recall that an ICBM can get almost anywhere on Earth in 45 minutes. Of course, populating many orbits with CAVs would reduce the response time, but that would also run up the cost. Aircraft carriers, submarines, and even CAVs launched on demand by Earth-based missiles would all provide better performance than a space-based CAV.

Another Objective laid out by the Rumsfeld commission was to defend existing military capabilities in space. While everyone agrees on the desirability of this goal, opinions vary over whether and how space weapons might help.

In framing the debate, it helps to consider the kinds of threats that existing satellites face. In roughly decreasing likelihood, these threats include: denial and deception (where an adversary conceals or camouflages its activities, hiding a chemical weapons lab within a mundane-looking agricultural fertilizer plant, for example, or using an underground bunker); electronic warfare (such as the jamming of satellite signals); physical attacks on satellite ground stations; blinding of satellite sensors with lasers; attacks in space by microsatellites; hit-to-kill antisatellite weapons; and high-altitude nuclear detonation.

Each threat would affect satellites differently. For instance, denial and deception thwarts only satellites performing intelligence-gathering missions. Satellites in geosynchronous orbit are less vulnerable to hit-to-kill weapons or a high-altitude nuclear burst. Other threats, such as electronic warfare and attacks on ground stations, could degrade the performance of all kinds of satellites.

Nor would space weapons be equally effective against these threats. Denial and deception, electronic warfare, attacks on ground stations, and satellite blinding--the four most likely threats--would be mounted predominantly from the ground, and space weapons would offer little or no defense against them. Moreover, these threats are low-tech and inexpensive compared with space weapons.

Space weapons might prove useful against microsatellites, antisatellite weapons, and nuclear explosions--attacks occurring in space and therefore more difficult to fend off from the ground. For example, a nuclear warhead detonated in space, even a warhead with one-100th the power of the 1.4-megaton hydrogen bomb that the United States tested at an altitude of 400 km in July 1962, would destroy or disable many of the hundreds of satellites in LEO [see Illustration, "Easy Prey"].

Easy Prey:

Hundreds of commercial, military, and research satellites now orbit relatively close by, in low-Earth orbit. Others lie in safer geosynchronous orbit, visible here as the ring of dots circling farthest from the Earth.

The blast wave from such an explosion would be insignificant, and even the powerful pulse of X-rays would affect only those satellites near the blast site. But many of the high-energy electrons from the products of nuclear fission would be trapped in the Van Allen radiation belts, degrading almost all satellites in LEO over the course of several months.

To initiate a high-altitude nuclear burst, a country must be willing to forgo its own space assets (or have few such assets to begin with). But the attack could do significant damage to valuable LEO satellites, including most military reconnaissance, surveillance, and intelligence satellites, as well as commercial and research satellites used for imaging and communication.

The means for such an attack already exist, in the form of thousands of Soviet-designed Scud missiles. The Scud-C, for example, sold by North Korea to Syria and other states, has a horizontal range of 600 km with a 700-kg payload; fired vertically, a Scud-C could reach 300 km. The positions of most large satellites are tracked by amateur astronomers and others and are readily available on the Internet. Accordingly, even a country with modest resources would be able to launch a Scud or some other short-range missile on a nearly vertical trajectory, arranged so that the apogee is in the path of an approaching satellite.

A single satellite in LEO can be destroyed without a nuclear warhead--if, for instance, a Scud used a mild explosive or a gas puff to disperse a few hundred kilograms of sand or gravel in LEO. The cloud of debris, falling only 1 km in the initial 15 seconds, would gravely threaten any satellite passing through it at orbital speeds of about 27 000 km/hr.

The threat that microsatellites could pose to existing space systems is probably greater than their potential benefit to the United States as weapons. An adversary microsatellite could use two quite different modes to destroy a quarry satellite. The first is direct impact: placed in an orbit that nearly intersects with its quarry's, the microsatellite could leisurely fire its booster rocket to convert a normal and nonthreatening 100-km miss into a direct collision.

Accelerating by just 0.1 km/s (an expenditure of 3 percent of the satellite's mass as rocket fuel) will net a 100-km displacement in 1000 seconds--about one-fifth of an orbit period in LEO, and far too little time for the quarry satellite's operators to take effective countermeasures. As the microsat approached the quarry, it might deploy a "lethality enhancement device," such as a net, to improve its chances of success. No short-range defense seems possible against such a high-speed intercept, unless the quarry satellite were capable of rapidly maneuvering out of harm's way, or unless it deployed confusion devices, such as balloon reflectors, to prevent the microsatellite from homing in on it. Current satellite systems are not known to have these protective capabilities.

A microsatellite could also launch an explosive or a projectile. For instance, the quarry would be unable to elude a space mine hovering just tens of meters away and equipped with an explosively driven pellet weapon or shaped-charge projectile. The microsat could also be programmed to fire if blinded or disturbed.

Various defenses to microsats can be imagined. A quarry satellite could be outfitted with sensors capable of detecting small, low-speed satellites, or it might be equipped with specialized defensive vehicles (perhaps even a fleet of bodyguard microsatellites of its own) to repel approaching space mines without harming the quarry.

How easy would it be to detect and track such space mines, and thereby thwart their attack? The U.S. Air Force Space Command, headquartered at Peterson Air Force Base in Colorado, indicates that it "is responsible for tracking objects larger than 10 centimeters orbiting Earth" and currently tracks some 9000 such objects.

But even perfect tracking would reveal only after the fact which satellite or launch was responsible for destroying the quarry. A real defense would require additional measures, such as those described above. And it is unclear, at least to us, how proposed U.S. space weapons would protect themselves against such threats.

If Space Weapons Are Not Our Best Hope protecting valuable communications, imaging, and other satellites, what are the alternatives? One attractive solution that avoids the political, economic, and technical difficulties of space weapons would be to reduce our dependence on space assets.

Satellite communication, for instance, typically relies on large and expensive satellites, and the loss of even one of these would have a crippling effect. Although some defense satellites do have backups, the majority of U.S. commercial communications and imaging systems have little redundancy. But if communications instead were configured in a distributed, load-balancing network of smaller satellites, an attack on one node, or even several, would do little harm. Such a strategy would also protect against system failures, accidents, and other disruptions to satellite communications.

As an alternative to redundancy and distribution, existing communications and intelligence-gathering satellites could be enhanced temporarily with terrestrial and airborne measures using unmanned aerial vehicles (UAVs), piloted aircraft, high-altitude balloons, or even rockets [see Illustration, "Networking on the Fly"].

Networking OnThe Fly:

High-altitude UAVs can supplement satellites during conflicts, relaying radio signals and intelligence imagery between headquarters and the battlefield.

These strategies might also arouse far less international opposition than would the deployment of space weapons. Such backup systems could also be more effective in local conflicts than the satellite system at risk.

Take the Global Positioning System, which currently consists of 28 satellites in medium Earth orbit. An adversary might have an interest in denying GPS capability in a particular locale--such as the battlefield--but rarely in denying the service worldwide. Also, it is far easier to jam the weak GPS signal across a few hundred kilometers than to destroy several of the GPS satellites in their higher orbits. In effect, a handful of jammers would do as much damage to local U.S. capability as the destruction of the satellites themselves.

Space weapons would be useless in countering such a scenario. Instead, within the expected area of jamming, the United States could deploy a network of short-range GPS transmitters carried by high-altitude UAVs, balloons, or, if necessary, rockets. Such "pseudolites," flying at altitudes of 20 to 30 km on UAVs or balloons, would use antennas to distribute a powerful GPS-like signal.

Pseudolites aboard sounding rockets, on the other hand, would have to be launched a few times a day to maintain a strong signal and would need large antennas to focus the energy on a small area. Either way, the pseudolites would effectively protect the real GPS network, because the enemy would not achieve its goal by destroying the satellites. In similar fashion, battlefield communications satellites could be replaced by radio relay transmitters aboard UAVs.

For imaging, UAVs could not only replace satellites but in many cases outperform their high-flying counterparts, as recent experiences in Iraq and Afghanistan have demonstrated. To begin with, UAVs can almost always get at least 10 times closer to an area of interest: a 20-cm mirror or lens on a UAV at 20 km above Earth would be equivalent to a 300-cm mirror aboard a satellite orbiting at 300 km. Furthermore, UAVs can linger over a site of interest, unlike satellites, and can carry a wider variety of imaging equipment, including optical, infrared, and advanced synthetic aperture radars, which can image through darkness and cloud cover. Beyond imaging, UAVs can readily track moving targets on the ground across an area of hundreds of kilometers.

On the other hand, satellites can and do provide global coverage that UAVs can never match. But most military operations are local. The real threats come from regional disruption, and those threats can be countered by regional alternatives.

Detonating a nuclear warhead in space would disable hundreds of satellites

Return Now To The Three Potential Roles for space weapons: protecting existing satellites, denying the hostile use of space, and projecting force worldwide. It is difficult to identify a space weapon that is more attractive than its competing terrestrial alternatives. Offensive space weapons face inherent limitations, including long distances to targets and high energy requirements, which suggest in many circumstances a non-space-based alternative, such as forward-deployed missiles and conventional ICBMs. In nearly every case, space weapons are more complex, more costly, and less effective than Earth-based weapons.

Moreover, we have seen that there are a number of ways to render military space systems inoperable without destroying the satellites themselves, such as attacks on their ground stations. In such cases, space weapons would be rendered useless. We have also argued that satellites could be better protected with redundant systems that would mitigate attacks or with stand-in capabilities provided by UAVs or balloons above the battlefield.

As for denying adversaries the use of space, this may likewise be more readily achieved by less expensive terrestrial alternatives, such as electromagnetic jamming and the temporary blinding of adversaries' reconnaissance systems.

The United States would prefer a world in which it alone had military space systems, weapons in space, and antisatellite capability. However, such a world never existed and never will. Already, several states and consortia have autonomous space-launch capabilities, among them Russia, China, Ukraine, Japan, India, and the European Union. Such groups would likely respond if the United States took a first step toward weaponizing space.

Consider, instead, a U.S. declaration that it would not be the first to deploy space weapons or to test destructive antisatellite systems, issued in parallel with an urgent challenge to negotiate an international treaty to this effect. From such a position, the United States could credibly declare that deploying space weapons would be regarded as a threat to U.S. security and that destruction of a U.S. satellite would be regarded as an attack on U.S. territory.

Even without space weapons, the United States could respond to an attack on its satellites with its unmatched terrestrial military capabilities. Adversaries would expect a heavy toll to be exacted as a result of any attack on U.S. satellites; that expectation alone would almost certainly suffice to deter any such attack.

In an all-out shooting war on Earth, we cannot expect that space would be a sanctuary for military systems supporting the weapons of that war. But the scenario sketched here, with the United States leading an urgent effort to ban space weapons and antisatellite tests or use, would help ensure that a shooting war on Earth would not be provoked by weapons in space.

This article opened with a fictional incident illustrating the appeal of space weapons. We will close by describing a possible outcome of such an incident, to offer a cautionary note about the risks and possible consequences of deploying space weapons.

12 June 2019--

On the one-year anniversary of the destruction of the command and control center of the rogue nation, a U.S. congressional review commission releases its findings. The center suffered minimal damage, returning to 75 percent capacity within 30 days, suggesting that the rogue country's leadership had been expecting such an assault. Additionally, no illegal weapons of any kind were found on the airliner in question. Several months after the incident, one of the six orbiting U.S. space-based laser satellites inexplicably exploded--causing an international space-debris incident of its own. This satellite happened to be the same one that destroyed the launch facility, having thus revealed its location. Suspicions include an adversarial space mine, but the orbiting clouds of debris tell no tales.

The final conclusion of the congressional commission: the rogue country's leadership instigated the incident by feeding the United States disinformation. The United States came away having disclosed its deployment of space-based weapons, to international outcry, and the incident was widely portrayed as U.S. bullying. While it is surmised that the smaller country had a hand in destroying a $20 billion U.S. satellite, its officials vigorously denied any role in the episode. In the end, the incident was recorded not as a measure of U.S. superiority in space but as a U.S. space debacle.

All Illustrations: John MacNeill

About The Authors

Bruce M. DeBlois is director of systems integration for BAE Systems, in Reston, Va. Richard L. Garwin (F) is IBM Fellow Emeritus at the Thomas J. Watson Research Center, Yorktown Heights, N.Y. (Correspondence should be addressed to him atRLG2@us.ibm.com.) R. Scott Kemp is a member of the research staff of the Program on Science and Global Security at Princeton University, in New Jersey. Jeremy C. Marwell is a Furman Scholar at the New York University School of Law, in New York City. This article is based on work the authors did while at the Council on Foreign Relations.

To Probe Further

For a similar classification of space weapons, see a report by Bob Preston et al., "Space Weapons, Earth Wars," published by RAND Corp., MR-1209-AF (2002) and available online at http://www.rand.org/publications/MR/MR1209/.